Hi Torsten, while I don't have a definitive reply, one of our native German-speakers said that "the top100de.txt list has probably a lot of duplicates and it does have some weird words in it, like ‘percent’, ‘million’, ‘Mark’ (Germany’s pre-Euro currency)."

Looking at this from a translator's perspective, I don't think there's a definitive answer to your question. A lot of translation is about making judgment calls, and in this case there isn't one objective, correct way to create a list of stop words — you can take a look at how they are discussed on Wikipedia to see what I mean: Stop words

The issue here is that the list of stop words is used to decide what words aren't included in searches. As a native speaker, you (and the other German speakers who contribute to GlotPress) are in the best position to determine what stop words will make searches more useful in German. If you think the list of stop words needs to be expanded, I don't see a problem with that — I think you should add as many stop words as you deem helpful and relevant. Cheers! :)

I already know the definition of Stopwords. You don't have to explain that to me. Thank you.
The best way to determine the best stopwords would be an analysis of the used search terms. But I don't even know for which search these stopwords are used. You wrote: "If you think the list of stop words needs to be expanded" - on which data should I rely? I have no data. And you are not able to provide it.

The quick answer seems to be: "the more the merrier". And that was so hard to tell...?